In devices designed to deliver and stack cut sheets, it is often desirable to provide jam detection in the event that sheets are improperly stacked or mislocated. Misstacking of sheets is of general concern where sheets are directed to a stack in overlapping or "shingled" orientation. In applicants' co-pending U.S. patent application Ser. No. 07/874,046, the contents of which are described herein, the sheets are directed from a source through a substantially horizontal guide into a vertical stack in a shingled orientation. In so directing sheets, a variety of possible jam conditions can develop.
FIGS. 28-30 schematically illustrate a vertical cut sheet stacker contemplated according to this invention. FIG. 28, particularly, illustrates a normal stacking process in which sheets 300 are driven between a pair of driving surfaces 302 into a vertical stack 304 located on a table 306.
As illustrated, sheets 300 are driven, according to this embodiment, in a correct shingled orientation in which the leading ends 308 of each sheet 300 are spaced from each other such that each leading end engages the upper stop 310 of the stacker at a discrete time. Hence, sheets are stacked in an orderly manner.
During the feeding process, the spacing between leading ends 308 can inadvertently change due to slippage between sheets, and fiber-lock interaction between sheets, so that the spacing or "shingle" between the leading ends 308 become improper. FIG. 29 illustrates a feeding error in which sheets 300 become bunched so that their leading ends 308 are substantially adjacent one another. In such an instance, binding and jamming can occur during stack formation, thus preventing sheets from entering the stack. Without proper jam detection, the feeder simply continues to deliver sheets to the feed rollers 302 forming a large multisheet jam that impedes further stacking.
Alternatively, sheets can become locked together by, for example, fiber-lock interaction and be driven past the stop 310 as depicted in FIG. 30. In such an instance, the leading ends 308 of sheets 300 extend over the top of the stack 304 and become creased at corners 312 against stop 310.
Again, without proper jam detection, the apparatus simply continues to deliver further upstream sheets 314 into the stack. The damaged sheets 300 become an undesirable part of the completed stack or, may impede driving of further upstream sheets 314 causing a jam adjacent the feed rollers 302.
Thus, it is desirable to detect the delivery of each sheet into the stack as it occurs. However, such detection has, in the past, been difficult. For example, if the sheets always overlap each other, then a "through beam sensor" or a mechanical switch that is triggered by each sheet, as it passes thereover, cannot be utilized. In addition, optical sensors can be easily fooled by different colors or by variations in brightness and shadows, or printing, on the sheets.
Many mechanical approaches jam detection, alternatively, are unreliable since there is very little mechanical evidence that a sheet has been delivered. Since each sheet only adds approximately 0.004 inch to the stack, and the stack is compressible, measurement of stack thickness is ineffective. Similarly, attempting to mechanically catch the edge of a sheet with a switch as the sheet slides past the sensor is difficult since the sheet edge is only 0.004 inch thick.
It is, therefore, desirable to provide a jam detection and registration system that verifies that each sheet has reached a predetermined "top stop" and has stopped at that point. In this manner, the system can detect when the sheet does not reach the stack or, alternatively, when the sheet is driven past the stop.
It is another object of this invention to provide a jam detection and registration system that accurately tracks the feeding of sheets to a selected location. The system should be capable of tracking sheets regardless of orientation and should track shingled and overlapping sheets.